1. Field of the Invention
The present invention relates to an active matrix substrate which constitutes a liquid crystal display apparatus or the like.
2. Description of the Related Art
FIG. 6 is a plan view showing an exemplary construction of a conventional active matrix substrate. FIG. 7 is a cross-sectional view taken along a line C-C' in FIG. 6.
In the active matrix substrate, gate bus lines 101 and source bus lines 102 which cross the gate bus lines 101 are formed on a transparent insulating substrate 110. In each of regions defined by the gate and the source bus lines 101 and 102, a picture element electrode 107 is formed. To the picture element electrode 107, a video signal is supplied from the corresponding source bus line 102 via a thin film transistor (hereinafter, abbreviated as a TFT) 103 as a switching element.
Referring to FIG. 7, the TFT 103 includes a thin silicon film 111 formed on the transparent insulating substrate 110, a gate insulating film 112 formed on the thin silicon film 111, and a gate electrode 113 formed on the gate insulating film 112. The thin silicon film 111 serves as a source electrode, a drain electrode and a semiconductor layer of the TFT 103. The gate insulating film 112 is formed so as to cover the thin silicon film 111. Then, the gate electrode 113 is formed on the gate insulating film 112. Over a substantially entire surface of the transparent insulating substrate 110 on which the gate electrode 113 has been formed, an interlayer insulating film 114 is formed.
A contact hole 104a is formed through the interlayer insulating film 114 and the gate insulating film 112. On the interlayer insulating film 114, the source bus line 102 (115 in FIG. 7) of a low resistive metal such as Al is formed. The source bus line 102 is connected to the source electrode of the TFT 103 via the contact hole 104a.
On the interlayer insulating film 114, a second interlayer insulating film 116 is formed. A contact hole 104b is formed through the insulating films 112, 114 and 116.
The picture element electrode 107 formed on the second interlayer insulating film 116 is made of an ITO (indium tin oxide) film. As is shown in FIG. 7, the picture element electrode 107 is connected to the drain electrode of the TFT 103 via the contact hole 104b.
As is shown in FIG. 6, an additional capacitance common interconnection 105 is formed parallel to the gate bus line 101. In an overlapping area where the additional capacitance common interconnection 105 overlaps the thin silicon film 111, an additional capacitance 106 is formed. In the additional capacitance 106, the additional capacitance common interconnection 105 serves as an upper electrode and the thin silicon film 111 serves as a lower electrode.
In the active matrix substrate with the above construction, when the potential of the gate bus line 101 is HIGH and the TFT 103 is in an ON state, a video signal is written into the picture element electrode 107 and the additional capacitance 106. The written signal is held under such conditions that the potential of the gate bus line 101 is LOW and the TFT 103 is OFF. Since the additional capacitance 106 is connected to the picture element electrode 107 in parallel, the signal holding property can be enhanced.
FIG. 8 is a cross-sectional view taken along a line D-D' in FIG. 6. As is shown in FIG. 8, in the prior art, the picture element electrode 107 overlaps the source bus line 102 for the following reasons.
In the case where there is any gap between the source bus line 102 and the picture element electrode 107, the gap portion does not contribute to a display. Moreover, it is necessary to provide a light blocking film on a counter substrate 119 for blocking light leaked from the gap. As a result, the opening ratio of the display apparatus is reduced by the gap and the light blocking film. In the case where the picture element electrode 107 overlaps the source bus line 102, the overlapping area does not contribute to a display, but an area which can contribute to the display can be maximum. In addition, in the overlapping portion of the picture element electrode 107, light is blocked by the source bus line 102, which means that light is not incident on the area which does not contribute to the display, so that a light blocking film for blocking leaked light therefrom is not required. As a result, the opening ratio of the liquid crystal display apparatus can be made larger, and the display apparatus can attain a brighter display.
In the conventional active matrix substrate having a structure in which the picture element electrode 107 is superposed on the source bus line 102, as is shown in FIG. 8, a step is made in the picture element electrode 107 because of the step of the interlayer insulating film 116 in the vicinity of the source bus line 102. As a result, the orientation of liquid crystal molecules is disordered in portions 108 of a liquid crystal layer 118 in the vicinity of the picture element electrode 107. The portion 108 is hatched in FIG. 8. The orientation disorder of liquid crystal molecules is increased due to an electric field around the bus line in addition to the geometrical factor of steps of the bus line.
The above-mentioned orientation disorder results in light leakage. Specifically, in a normally white mode display, there occurs a phenomenon that the vicinity of the step of the bus line is white even when a voltage is applied to the liquid crystal for a black display. As a result, the contrast ratio of the display apparatus is degraded, and hence the display quality is deteriorated.